This cornice is made using curves and having a profile as the curves bevel shape.

View this tutorial on how to use them to drastically speed up your workflow! https://youtu.be/y55OJlzXGlY

To use it follow these steps:
1. Create the room shape
2. Select the whole edge around the floor of your room and duplicate it, make it its own object
3. Convert the object to a curve
4. Join the curve to one of these skirting profiles you download from here